Image forming apparatuses such as electrophotographic copying machines and laser beam printers typically adopt a heat fixing method that forms an image of toner on recording paper with an image forming process and thermally fixes the image. The toner is made of, for example, heat-melting resin. The image forming process includes electrophotography, electrostatic recording, and magnetic recording.
Such conventional heat-fixing methods involve a roller for fixing or pressuring. The roller often includes a metal core shaft made of for example, aluminum, a rubber layer on the shaft, made of, for example, silicone rubber, and a fluorine resin layer on the rubber layer, made of fluoroplastics with good toner releasability, such as a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) or polytetrafluoroethylene resin (PTFE). In recent years, a roller has been increasingly used which have a rubber layer formed of foam rubber (sponge rubber), in particular, open-cell foam rubber, from viewpoints of sufficient nip width, energy saving, a reduction of volatile organic compounds (VOC). The open-cell foam rubber has higher air permeability and prevents the outside diameter and the surface hardness of the roller from varying due to a rise in temperature of the roller in operation. The variation of the surface hardness and the outside diameter vary a fixed image or a paper feed speed.
Known methods for expanding rubber, that is, for producing sponge rubber include (i) a method involving a heat foaming agent, (ii) a method involving a hollow filler, and (iii) an elution method.
Among these methods, the method (i) turns the foaming agent into cracked gas at a stage where the rubber is cross-linked, to generate bubbles, thereby producing the sponge rubber. Unfortunately, the cracked gas may cause smell. Further, the foaming agent may inhibit the curing of a rubber material when the rubber material is addition-reaction silicone rubber cured with a platinum catalyst.
The method (ii) does not easily form a uniform cell (bubble) structures because of a large density difference between the hollow filler and the silicone rubber. Further, the manufacturing cost may increase because the filler is not commonly used.
The elution method (iii) mixes water-soluble powder in the rubber, and after cross-linking the rubber, elutes the water-soluble powder into water or hot water to produce the sponge rubber. Unlike the method (i), this method does not involve the heat foaming agent. In addition, this method does not involve uncommon materials unlike the method (ii), which can decrease the manufacturing cost. Conventional elution methods, however, possibly forms a rubber skin layer (thin layer) between particles of the water-soluble powder. This layer prevents sufficient elution of water-soluble powder to obstruct formation of sponge having good uniformity and a high open-cell ratio for heat fixing.